Method and apparatus for creating pulsatile flow in a cardiopulmonary bypass circuit

ABSTRACT

Pulsatile cardiopulmonary flow closely approximating a natural heartbeat is generated in a heart-lung machine by using a proportioning valve to partly convey blood in the machine&#39;s cardiopulmonary circuit to the arterial supply line, and to partly recycle that blood into the cardiopulmonary circuit upstream of the arterial pump. The relative proportions of conveying and recycling are varied in accordance with a waveform approximating the waveform of a human heartbeat.

FIELD OF THE INVENTION

This invention relates to heart-lung machines, and more particularly toa method and apparatus for simulating the natural heartbeat's pressurepattern in the blood output of the heart-lung machine.

BACKGROUND OF THE INVENTION

The natural human heart provides the body with a pulsatile flow of bloodcorresponding to the filling and emptying (beating) of the variouschambers of the heart. The instantaneous blood flow rate varies in acomplex cyclical manner from near zero to some maximum rate, with theoverall blood flow rate being a time weighted average.

The cardiopulmonary bypass circuits of heart-lung machines used inopen-heart surgery typically utilize centrifugal or positivedisplacement (i.e. roller type) pumps to provide the motive power forcirculation of the blood. These pumps provide an essentially constantflow rate of blood through the circuit at all times, the instantaneousrate and the average rate being nearly identical.

Medical studies have suggested that pulsatile flow, being morephysiologically correct than constant flow, may have a beneficial impacton the efficacy of the extracorporeal perfusion. This can result inimproved patient outcomes following cardiac bypass surgery.

Various ways have been proposed to mimic in a heart-lung machine thenatural pulsatile flow of the heart, but none of them have so far beensatisfactory. The simplest way of providing a pulsed flow is tocyclically clamp and unclamp the inlet or outlet line of the heart-lungmachine's arterial pump. Clamping the pump inlet is not desirable sinceit can create very high suction pressures in the inlet which can damagethe red blood cells, or in some cases even cause cavitation which canpotentially release gas bubbles into the blood stream. Further, duringthe low flow or rest periods, the pump rotors spin on a stagnant volumeof fluid, which may result in mechanical trauma to the blood cells.Clamping the pump outlet is not desirable in a centrifugal pump due tothis mechanical trauma. Clamping the pump outlet is not desirable in apositive displacement pump since the rapid buildup of pressure in thelines can rupture the connections or tubing, potentially resulting in acatastrophic event.

A more acceptable way of creating pulsatile flow is to vary the speed ofthe pump in a cyclical manner. This is easily accomplishedelectronically by the pump controller. However, the inertia of thespinning elements of the pump tends to render the resulting waveformmore sinusoidal than the natural heartbeat waveform and forces the waveperiod to be longer than the natural period. In addition, the componentsof the bypass circuit downstream of the pump, such as the oxygenator andarterial filter, also damp the pulses due to their volumetric holdup.

Lastly, a reciprocating type pump such a a diaphragm or bladder pump canbe employed to create pulses in the flow. These pumps tend to be moremechanically complex than the roller or centrifugal types and do notlend themselves to either easy cleaning, sanitation, and sterilizationfor reuse, or low cost manufacture for one-time disposable use.Increased blood trauma is experienced in these pumps due to the multiplecheck valves in the flow path and stagnant areas due to less thanperfect chamber filling and ejection. Lastly, as mentioned above,downstream components still damp the pulses and thus reduce thebeneficial effects of the reciprocating pump.

SUMMARY OF THE INVENTION

The present invention provides an undamped pulsatile blood output in aheart-lung machine that is a very close approximation of the pulsationof the human heart by placing an automatic proportioning valvedownstream of the oxygenator, filter, and other components in thecardiopulmonary circuit of the heart-lung machine. Pulsatile flow isachieved by cyclically switching the blood flow between the arterialsupply line and a recycle line which returns the blood to a pointupstream of the arterial pump. This eliminates the damping effect of thevarious components in the bypass circuit. Since a proportioning valveonly redirects flow and never actually stops it, the device can be usedwith both positive displacement and centrifugal pumps without fear ofcavitation, overpressurization, stagnation, excessive blood trauma orshear heating.

The inventive system consists of a single use (i.e. disposable) plasticproportioning valve connected to a reusable electric or pneumaticactuator and control console. Additional inputs to the console can befrom a flow meter, pump speed controller, air-in-line detector or otherexternal signal.

The inventive valve can be sterilized and supplied either pre-connectedinto the bypass circuit or as a stand alone component which may beinserted into the circuit in the field. Although traditional plug typeproportioning valves are acceptable, a low pressure drop, low shearvalve design such as a diaphragm type valve, produced from biocompatiblematerials, is preferred since this will minimize any mechanical traumato the blood.

The function of the controller is to provide the user interface to setthe periodicity, amplitude, and waveform of the pulses and to sendappropriate control signals based on the setpoints to the valve actuatorto effect the pulsatile flow. If desired, the controller can utilize theadditional inputs mentioned above to perform additional controlfunctions.

Periodicity of the pulses is controlled by the rate of valve cycling andcan be set by the user. Since the inertia of the valve stem isrelatively small, rapid cycling of the valve is easily accomplished.This is a clear advantage over systems utilizing pump speed as the meansof effecting pulsatile flow. If capability for external triggering isbuilt into the controller, the patient's own heartbeat can be used topace or set the cycling of the valve. This would be useful at the end ofthe surgical procedure to aid in weaning the patient from the heart-lungmachine.

The waveform of the pulses is controlled by the rate of change of theposition of the proportioning mechanism which can be set by the user.Slow movement will create a sinusoidal waveform, rapid movement willcreate a square waveform, and complex movement patterns can createwaveforms nearly identical to the human pulse. The ability to create anywaveform is a clear advantage over the prior art systems which utilizeocclusive clamps or pulsatile pumps to create the pulsatile flow.

The amplitude of the pulses is controlled by the relative proportion ofthe total flow through the supply line. This can vary from no flow (100%recycle) to full flow (100% supply), or any combination in between. Thisparameter can be set by the user.

If an air-in-line detector is placed an appropriate distance upstream ofthe valve, the system can act as an automatic air eliminator. When airis detected, the valve can go into 100% recycle mode which will divertthe air back upstream where it will be eliminated through the oxygenatoror caught in the bubble trap on the arterial filter. The diversion timecan be pre-set to some value, or the valve may remain in recycle modeuntil manually switched back into the pulse mode.

An averaging flow meter is preferably placed downstream of the valve toprovide information regarding the actual flow rate to the patient. Theaveraging function is necessary due to the fluctuations in flow ratecaused by the pulsing flow. The controller can readily be configured toprovide feedback to the arterial pump to maintain a desired average flowrate.

Another benefit of the inventive system is that due to the recycle loop,the average residence time of the blood in the oxygenator is increased.This increases the likelihood that the blood will be completelyoxygenated before being transferred back to the patient. In addition,the recycled blood helps to pre-cool or pre-heat the incoming blood,which reduces the load on the heat exchanger in the oxygenator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a heart-lung machine using the invention;and

FIG. 2 is a block diagram of the controller of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the heart-lung machine 10 of this invention connected to apatient 12. During surgery, blood is diverted from the patient's venacava 14 and conveyed to a venous reservoir 16. The reservoir 16 mayconventionally contain a cardiotomy filter 18 through which bloodscavenged from the surgery field by the cardiotomy pump 20 and sucker 22can be returned to the cardiopulmonary bypass circuit 24.

In the cardiopulmonary circuit 24, blood taken from the reservoir 16 iscontinously pumped by the arterial pump 26 through the blood oxygenator28 and the arterial filter 30. In conventional heart-lung machines, theoutput of arterial filter 30 is discharged directly into the arterialsupply line 32 which returns the oxygenated blood to the aorta 34 ofpatient 12.

In accordance with the invention, however, the output of arterial filter30 is preferably conveyed through an air-in-line detector 36 to aproportioning valve 38 operated by a pneumatic or electric actuator 40under the control of a controller 42. The proportioning valve directsvarying proportions of the blood flow from line 44 into the arterialsupply line 32 through an averaging flow meter 46, and into the recycleline 48 as determined by the actuator 40. The recycle line 48 dischargesrecycled blood into the cardiopulmonary circuit 24 upstream from thearterial pump 26, and preferably into the venous reservoir 16 forreasons discussed below.

The operation of controller 42 is shown in more detail in FIG. 2. Inthat figure, the controller 40 is shown to contain a microprocessor 50which is under the control of predetermined operator settings 52, andwhich receives input signals representative of the average flow rate inthe arterial supply line 32 as measured by the averaging flow meter 46;the actual speed of arterial pump 26; and any air in line 44 detected byair-in-line detector 36. If desired, a further input signalrepresentative of the actual beat of the heart 54 of patient 12 may alsobe supplied to the microprocessor 50.

A wave generator 56 is provided to produce a waveform simulating thenatural beat of the patient's heart 54, either from digital datagenerated by the microprocessor during surgery (either on its own orfrom pre-surgical data received from heart 54), or, at the direction ofthe operator, in synchronism with the patient's actual heartbeat afterthe heart 54 has been restarted following surgery.

The output of wave generator 56 is applied to the pneumatic or electricactuator drive 58 which controls the actuator 40 of the proportioningvalve 38. The wave input to actuator drive 58 may be momentarilyoverriden by an air-in-line signal from microprocessor 50 when an airbubble is detected in line 44 by the air-in-line detector 36. In thatcase, the microprocessor 50, knowing the speed of pump 26 and the lengthof the line 44 between the air-in-line detector 36 and the proportioningvalve 38, can switch the actuator drive 58 to 100% recycle at the rightmoment and just long enough to divert the air bubble into recycle line48 without significantly affecting the blood output into arterial supplyline 32.

The microprocessor 50 also controls the speed of pump 26 as necessary tomaintain the average flow rate measured by the flow rate meter 46 at thelevel set by the operator at 52. For monitoring purposes, the output ofactuator drive 58 may be displayed on a display 60.

The output of recycle line 48 may be fed into the input of the arterialpump 26 or into the venous reservoir 16. The latter is preferablebecause it helps to prevent stagnation by increasing the turnover in thereservoir 16. In addition, if air bubbles in the line 44 areautomatically diverted into the recycle line 48 as described above,discharge of the recycle line 48 into the reservoir 16 allows the airbubbles to be vented to atmosphere.

It is understood that the exemplary cardiopulmonary bypass circuitdescribed herein and shown in the drawings represents only a presentlypreferred embodiment of the invention. Indeed, various modifications andadditions may be made to such embodiment without departing from thespirit and scope of the invention. Thus, other modifications andadditions may be obvious to those skilled in the art and may beimplemented to adapt the present invention for use in a variety ofdifferent applications.

I claim:
 1. A heart-lung machine, comprising:a) a venous reservoir forreceiving venous blood from a patient; b) an arterial supply line forconveying oxygenated blood to said patient; c) a cardiopulmonary circuitincluding said reservoir, said arterial supply line, and an arterialpump and an oxygenator interposed between said reservoir and saidarterial supply line in said circuit; d) a recycle line connected todischarge blood into said circuit upstream of said pump; and e) aproportioning valve interposed in said circuit downstream of said pumpand upstream of said arterial supply line and recycle line, said valvebeing arranged to convey selected varying proportions of the bloodpumped by said arterial pump to said arterial supply line and lo saidrecycle line.
 2. The heart-lung machine of claim 1, in which said valveis connected between said oxygenator and said arterial supply line. 3.The heart-lung machine of claim 2, in which an air-in-line detector isinterposed in said circuit upstream of said valve.
 4. The heart-lungmachine of claim 2, in which an averaging flow meter is interposed insaid circuit between said valve and said arterial supply line.
 5. Theheart-lung machine of claim 1, in which said recycle line is connectedto discharge blood into said reservoir.
 6. The heart-lung machine ofclaim 1, in which said proportioning valve is a diaphragm valve.
 7. Theheart-lung machine of claim 1, further comprising a controlleroperatively connected to said porportioning valve and arranged to varysaid proportions in accordance with a predetermined waveformsubstantially simulating the waveform of a human heartbeat.
 8. Theheart-lung machine of claim 7, in which said arterial pump is operableat variable speeds, and in which said controller is further arranged tocontrol the speed of said arterial pump in such a manner as to maintaina predetermined average blood flow rate in said arterial supply line. 9.The heart-lung machine of claim 7, in which an air-in-line detector isinterposed in said circuit upstream of said valve, and said controlleris further arranged to switch said valve to 100% recycle when a detectedair bubble reaches said valve.
 10. The heart-lung machine of claim 7, inwhich said patient has a natural heartbeat, and in which said controlleris further arranged to either selectably generate a waveform for varyingsaid proportions, or to vary said proportions in substantial synchronismwith said heartbeat of said patient.